The present invention relates to a lagged flexible pipe for conveying a fluid such as a gas, crude oil, water or other fluids over long distances. The invention is quite particularly aimed at such a pipe intended for offshore oil production.
EP 0,400,689, in the name of the assignee company, for example, discloses a flexible pipe of this type, produced by helically winding at least one strip of insulation around a flexible central core. Each strip is formed of an extruded section of the insulation, and in practice, a sufficient number of layers of strip is laid in order to obtain the required insulation. An outer sealing sheath surrounds the thermal insulation.
The strip is laid at an angle of close to 90.degree. with respect to the axis of the pipe, and a space between the turns, also known as a gap, is needed so that the pipe can be wound to its minimum radius of curvature without increasing its stiffness in bending. The size of the gap between the turns is typically about 10% of the width of insulating strip. If there is no gap,the insulating layer would behave like a continuous sheath of the material used to manufacture the said strip, and this would considerably increase the stiffness: thus, while the stiffness of a pipe with a lagging winding with a 10% gap is, in a particular example, 9.4 kN/m.sup.2, the stiffness of the same pipe made with continuous winding of the same insulation would rise to 26 kN/m.sup.2.
Industrially, the materials used for this type of application need to exhibit an optimum combination of thermal and mechanical properties. Thus, for example, a low coefficient of thermal conductivity (lambda) and a compression strength compatible with the hydrostatic pressure generated by the head of water in which the pipe is laid are combined. Typically, good materials have a lambda value of the order of at most 0.15 W/m.K, a compressive stress between plates in excess of 10 MPa at 90.degree. C., a modulus in tension in excess of 600 MPa at 23.degree. C., and an elongation at the threshold of plastic deformation in excess of 7% at 23.degree. C.
Although the above pipe is generally satisfactory, it has been found that the gap needed between the turns of the insulating strip gave rise to two kinds of problems. On the one hand, when the outer sealing sheath becomes torn, sea water can enter the free volume formed by the gap and thus increase the lambda value. On the other hand, the combination of the hydrostatic pressure and of the temperature when the pipe is in service may cause the material of the insulating strip to creep into the gap between turns, and this results in an increase in the apparent modulus of the layer considered, an increase in the bending stiffness of the pipe, and a reduction in the thickness of the insulating layer, leading to a drop in the thermal-insulation performance of the flexible structure.
A solution which would consist in winding without a gap an extruded section which combines all the thermal and mechanical requirements (mainly, typically: a lambda value of the order of at most 0.15 W/m.K, a compressive stress between plates in excess of 10 MPa at 90.degree. C., a modulus in tension of below 100 MPa at 23.degree. C., and an elongation at the threshold of plastic deformation in excess of 7% at 23.degree. C., is not possible as, to the applicant's knowledge, no extrudable material which exhibits these properties yet exists.